It is shown in the United States patent application of P. J. Mallozzi, H. M. Epstein, R. G. Jung, D. C. Applebaum, B. P. Fairand, and W. J. Gallagher, for Producing X-Rays, Ser. No. 319,756, filed Dec. 29, 1972 now U.S. Pat. No. 4,058,486, issued Nov. 15, 1977, that an intense point source of X-rays can be generated by focusing a laser beam onto a solid target. Neodymium laser light focused onto a solid slab target has been converted into X-rays with an efficiency greater than 25 percent, with several tens of joules of X-rays emanating from an essentially point source (about 100 microns diameter) in a nanosecond. The X-ray pattern produced with iron targets irradiated with about 100-joule laser pulses at a 45 degree angle of incidence is substantially omnidirectional. The conversion efficiency of greater than 25 percent refers to X-rays which are radiated away from the slab and pass perpendicularly through 3000 Angstroms of plastic (paraline) coated with 2000 Angstroms of aluminum. This conversion efficiency is thus a lower bound and refers only to the portion of the spectrum above about 300 electron volts. Most of the observed X-rays lie between about 0.3 and 1.5 keV, with a small but useful fraction having energies as high as 10 to 100 keV. In a densitometer tracing of a bent crystal spectrograph taken with a KAP crystal, the radiation appears to be mostly lines in the spectral interval of about 0.7 to 1.2 keV. The unusual sharpness of the spectral detail is due to the tiny dimensions of the source.
This novel point source of X-rays provides a spectrum tuneable throughout a range of about 0.1 to 100 keV. This can provide hospitals with revolutionary new X-ray machines for taking ultrasharp, ultrafast radiographs, and for performing swift and precise irradiations of cancer tissue. Indeed, the applications to medical research, diagnosis, and treatment seem virtually endless. A few of the more important applications according to the present invention are discussed below.
In one typical application the soft X-ray output is used to take X-ray photographs. For example, a 8 mm thick section of a dog's heart and a 6 mm thick cancer nodule (carcinoma 255) were taken by placing dessicated samples over dental film at a distance of about 10 cm from the focal spot. Comparison with a radiograph of the same dog heart section taken with a conventional high resolution soft X-ray source (the Picker Hotshot) using a 1 minute exposure shows that the laser photograph not only matches the high resolution of the conventional photograph, but also has the added advantage of a pulse width that is short enough to arrest any biological motion that might occur. The short pulse width of the X-rays makes them also especially adaptable to flash microradiography, with a resolving power of less than a micron being possible.
Another application of the new X-ray source is the soft X-ray irradiation of tumors located on the skin or on the surface of internal organs, thereby effecting the selective destruction of the tumor without injuring underlying tissue. The selective irradiation is made possible by the short mean-free-path of soft X-rays in tissue. A second useful property of soft X-rays (as opposed to hard X-rays) is their large critical angle for total reflection when incident on smooth surfaces such as float glass, etc. The critical angle for float glass is approximately 1.5 degrees at 1 keV, and can be three or four times higher for smooth surfaces of high-Z materials such as tungsten, gold, and platinum. This reflection property has permitted the development of a new device that channels X-rays in much the same way that light is guided by a light pipe, and may conveniently be described as an "X-ray pipe." New types of endoscopes can be constructed based on the X-ray pipe principle. One such device is essentially a conventional bronchoscope except that it contains a glass tube about 8 mm in diameter for channeling the X-rays. A straight piece of tubing about 107 cm long was tested with soft X-rays and was found to deliver about 32 times more Joules per square centimeter than were delivered in a purely line of sight irradiation. A similar experiment was performed with a curved tube, in which case the enhancement dropped to a factor of about 8. Other typical X-ray pipe devices include an "X-ray hypodermic needle" that can be inserted into a subject in the same way as a conventional hypodermic needle is inserted. Another is an "X-ray catheter" consisting of a flexible X-ray pipe that can be run through the arteries to remote locations such as the interior of the heart. These devices have the capability of channeling the X-rays around corners. For sharp angles, where only very soft X-rays can survive the turn, Bragg angle reflection is employed.
The hard X-rays produced with the laser can provide sharper radiographs of the human body, including chest X-rays that stop cardiovascular and respiratory motion, for example. A hard X-ray probe for taking radiographs from special vantage points in the body comprises typically a long narrow hollow cone with a special target on its tip. The cone is inserted in the body and the laser beam is focused through the cone to the target by means of a long focal length lens.
Aside from the radiography applications, the hard X-rays can also play a role in radiotherapy, utilizing the probe described above (which can also deliver soft X-rays), or Bragg angle, critical angle, or Fresnel lens focusing devices can direct the X-rays to small points in the body.
Further details of typical embodiments are provided in the drawings and in the detailed description herein.